High-performance diode device structure and materials used for the same

ABSTRACT

A diode and memory device including the diode, where the diode includes a conductive portion and another portion formed of a first material that has characteristics allowing a first decrease in a resistivity of the material upon application of a voltage to the material, thereby allowing current to flow there through, and has further characteristics allowing a second decrease in the resistivity of the first material in response to an increase in temperature of the first material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/580,013, filed on Oct. 15, 2009, the subject matter of which isincorporated in its entirety by reference herein.

FIELD OF THE INVENTION

Disclosed embodiments relate generally to diode devices and moreparticularly to high-performance diode devices exhibiting favorableforward current characteristics.

BACKGROUND

Diodes are one of the most fundamental semiconductor elements orcomponents. A diode usually allows current flow in one direction but notin another. A diode is constructed as a two-region device separated by ajunction; however, various types of diodes which have different junctionstructures also exist. Two examples of common diode types includesilicon-based doped p-n junction diodes and Schottky diodes.

Diodes have many applications and are frequently used in, for example,memory devices, logic circuits or solar cells, or may function as LEDs(light emitting diodes). Diode devices with high ON current and highON/OFF current ratio (resulting in low leakage current) also have manyapplications, such as for example, as a select device in a memoryelement. Silicon-based junction diodes may provide high ON current andhigh ON/OFF current ratios, but the manufacturing process ofconventional silicon-based junction diodes is much more complicated andrequires higher processing temperatures than, for example, ametal-insulator-metal (MIM) diode system. Unfortunately, known MIM diodesystems are not able to meet the high forward current requirements ofmany applications.

Accordingly, there is a need and desire for a diode device that exhibitsfavorable forward current characteristics while also being easilymanufactured at low temperatures (<500° C.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a diode according to a disclosedembodiment.

FIG. 2 is a flowchart illustrating a manufacturing method for a diodeaccording to FIG. 1.

FIG. 3 is an I-V curve of a diode according to a disclosed embodiment.

FIG. 4 is a cross-sectional view of a diode according to anotherdisclosed embodiment.

FIG. 5A illustrates a cross-sectional view of a cross point memorydevice including a diode according to a disclosed embodiment.

FIG. 5B illustrates a top view of the cross point memory device of FIG.5A.

FIG. 6 is an I-V curve of a diode included in a memory device accordingto a disclosed embodiment.

FIG. 7 illustrates a processing system, utilizing a diode and/or memorydevice according to a disclosed embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which are shownby way of illustration specific embodiments that may be practiced. Itshould be understood that like reference numbers represent like elementsthroughout the drawings. These example embodiments are described insufficient detail to enable those skilled in the art to practice them.It is to be understood that other embodiments may be utilized, and thatstructural, material, and electrical changes may be made, only some ofwhich are discussed in detail below.

Disclosed embodiments relate to a diode device and methods ofconstructing and operating the same, wherein the diode exhibits high ONcurrent as well as a high ON/OFF ratio. Disclosed embodiments achievethese characteristics by using a combination of the properties of lowelectric field turn-on and joule assisted self heating to build highforward currents at relatively low voltage values. The embodimentsutilize a material that has a first characteristic allowing a firstdecrease in a resistivity of the material upon application of a lowvoltage and a second characteristic allowing a second decrease in theresistivity of the material in response to an increase in temperature ofthe material.

One disclosed embodiment is illustrated in FIG. 1. Diode 10 a includesconductive materials 15 and 25 on either side of material 20 (describedbelow in more detail). Conductive materials 15 and 25 may also bereferred to as diode electrodes. Referring to FIG. 2, the exampleembodiment of FIG. 1 may be fabricated as follows. Conductive material15 may be formed over any suitable substrate base (not shown), at stepS1. Conductive material 15 may be patterned as desired by utilizingphotolithographic processing and one or more etches, or by any othersuitable patterning technique. Material 20 may then be formed overconductive material 15, at step S2. In some embodiments, material 20 maybe deposited on material 15 and may then be patterned usingphotolithographic processing and one or more etches, or by any othersuitable patterning technique. Material 20 may be deposited with anysuitable methodology, including, for example, atomic layer deposition(ALD) methods or plasma vapor deposition (PVD) methods, such assputtering and evaporation, thermal deposition, chemical vapordeposition (CVD) methods, plasma-enhanced (PECVD) methods, andphoto-organic deposition (PODM). Conductive material 25 may then bedeposited over material 20, at step S3, using one or more of thetechniques described above in relation to material 20, or any othermethod. Each of conductive materials 15 and 25 and the material 20 maybe formed as thin-films. Alternatively, diode 10 a may be formed bylayering all of the materials 15, 20 and 25 and then etching them all atonce to form a diode stack.

The above-described method is much simpler than the complicated stepsrequired for forming, for example, a conventional silicon-based junctiondiode. Additionally, the manufacturing process of conventionalsilicon-based junction diodes requires higher processing temperaturesthan those of the disclose embodiments, which have a thermal budget ofless than 500° C.

Conductive materials 15 and 25 may include any suitable conductivematerial, such as, for example, one or more of various metals, such astantalum, platinum, tungsten, aluminum, copper, gold, nickel, titanium,molybdenum, etc., metal-containing compositions, such as metal nitrides,metal silicides (e.g., tungsten silicate or tantalum silicide, etc.),and conductively-doped semiconductor materials (for instance,conductively-doped silicon). Additionally, conductive materials 15 and25 may be formed of the same material or of different materials.

Material 20 is selected such that under initial programming (off)conditions, current is not conducted from material 25 to material 15,but that under appropriate operating conditions, the material 20undergoes a transition and becomes conductive. Appropriate materialsselected for material 20, in accordance with disclosed embodiments, arethose that operate in a fashion similar to the operation illustrated inthe I-V curve of FIG. 3. There are two separate phenomena at work inconjunction with each other to allow the particular curve profile shownin FIG. 3 to be achieved. These phenomena are low electric fieldassisted metal-insulator transition and Joule heating assisted currentincrease.

Low electric field assisted metal-insulator transition occurs inmaterials whose resistance is changed when a voltage is applied to thematerial. While the voltage remains below a threshold voltage value(V_(th)), material 20 has a high resistivity. When the voltage reachesthe threshold voltage value (V_(th)), however, material 20 quicklychanges to a low resistivity. Joule heating assisted current increaseoccurs when, due to the temperature increase of the material resultingfrom the heat generated by the current flowing through the material, theresistivity of the material suddenly decreases, allowing a correspondingsudden increase in current flow.

The disclosed embodiments select materials for material 20 which embodyboth of these phenomena. As illustrated in the I-V curve of FIG. 3, anappropriate material for material 20 will have a low threshold voltage(V_(th)) and allow high forward current (A) to the diode at relativelylow voltages. As voltage V is applied across diode 10 a, only leakagecurrent is present until the threshold voltage V_(th) is reached. Atthat point, a steep increase 30 in the current occurs. This increase isdue the low electric field assisted metal-insulator transition effect.Small electric fields (few kV/cm) across the diode structure 10 a willinduce an insulator-metal transition in material 20, causing the abruptincrease 30 in current seen at the threshold voltage V_(th). Indisclosed embodiments, the threshold voltage preferably occurs at avoltage value of approximately 0.5V to approximately 1.5V and this ispreferably observed at room temperature.

As voltage continues to be applied, the temperature of material 20 willincrease, due to the resistance of the material to the current flow.Once the material reaches a threshold temperature, the Joule heatingassisted current increase will be seen, such as that shown in the steepincrease 35 in the current flow. In disclosed embodiments, the Jouleheating assisted current increase preferably occurs at voltage values ofapproximately 4.0V or below (e.g., between approximately 1.5V andapproximately 4.0V).

FIG. 3 should not be taken as a precise representation of an I-V curveaccording to disclosed embodiments but should only be taken as anapproximation of a representative typical temperature profile accordingto one the disclosed embodiments. FIG. 3 should not be interpreted as alimitation on the scope of all embodiments.

According to disclosed embodiments, this combination of multiple steepcurrent increases allows very high forward current drive at a relativelylow voltage and a relatively low current at voltages below the thresholdvoltage, resulting in a high performance diode device having an ultrahigh forward current density (e.g., between approximately 1×10⁶ A/cm² toapproximately 1×10⁸ A/cm², and preferably on the order of 1×10⁷ A/cm²)and a very high ON/OFF current ratio. Additionally, the I-V curves ofthe diode of the disclosed embodiments are non-ohmic and the change inthe resistivity over the curve can be several orders of magnitude(>10⁵).

In order to achieve such a beneficial result from the low electric fieldassisted metal-insulator transition and Joule heating assisted currentincrease, material 20 must be chosen to be compatible with each of theseeffects. Some materials found to be particularly suited for such anapplication include, for example, vanadium oxides, particularly VO₂,aluminum-doped vanadium oxides, titanium-doped vanadium oxides, or rareearth manganates (Ln_((1-x))A_(x)MnO₃, Ln=rare earth, A=alkaline earth),particularly Sr_((1-x))La_(x)MnO₃, where 0.0≦x≦0.4.

In another embodiment, a heating element may additionally be included inthe diode in order to assist with the temperature increase, thus moreeasily reaching the temperature at which the current increases for thesecond time. This is illustrated in the diode 10 b of FIG. 4, whichfurther includes heating element 28 between electrode 15 and material20. Alternatively, the heating element 28 may be positioned betweenmaterial 20 and electrode 25 or two heating elements 28 may be included,one positioned between electrode 15 and material 20 and the otherpositioned between material 20 and electrode 25. Heating element 28 isformed of any material having a suitable resistance to cause the desiredamount of heating when current flows through the material while stillallowing current flow through the diode, such as for example metaloxides. Heating element 28 may also be, for example, a metal heatingcoil. Other elements of FIG. 4 are the same as described with respect toFIG. 1.

Diodes of the disclosed embodiments may be used in any applicationrequiring a diode, and in particular one in which high forward currentdensity is desired. Examples include memory applications, logiccircuits, solar cells, or LEDs. One example application particularlysuited to the diodes of the disclosed embodiments is a select device fora cross point memory device 100, as illustrated for example in FIGS. 5Aand 5B.

FIG. 5A illustrates a cross-sectional view of a cross point memorydevice 100 including a diode 10 a (FIG. 1). Alternatively, diode 10 b(FIG. 4) may be used in place of diode 10 a. FIG. 5B illustrates a topview of the cross point memory device 100. The access lines 110, forexample word lines, and data/sense lines 120, for example bit lines, ofthe cross point memory device 100 are connected at their intersectionsby a diode 10 a of the disclosed embodiments. In the cross point memorydevice application of the diode 10 a of the disclosed embodiments, wordline 110 operates as the diode electrode 15 (FIG. 1) and electrode 150operates as diode electrode 25 (FIG. 1). Memory element 140 is presentabove electrode 150 and is accessed via the diode 10 a. As illustratedin FIG. 6, after the first increase in current, the memory element 140may be “read” and after the second increase in current, the “write”function may be performed.

The diode 10 a, 10 b and/or memory array 100 may be fabricated as partof an integrated circuit. The corresponding integrated circuit may beutilized in a processor system. For example, FIG. 7 illustrates asimplified processor system 700, which includes a memory device 702which may include memory array 100 (and diode 10 a, 10 b) in accordancewith the above described embodiments. A processor system, such as acomputer system, generally comprises a central processing unit (CPU)710, such as a microprocessor, a digital signal processor, or otherprogrammable digital logic devices, which communicates with input/output(I/O) devices 720 over a bus 790. The memory device 702 communicateswith the CPU 710 over bus 790 typically through a memory controller. Theprocessor system 700 may also include flash memory 760.

In the case of a computer system, the processor system 700 may includeperipheral devices such as removable media devices 750 (e.g., CD-ROMdrive or DVD drive) which communicate with CPU 710 over the bus 790.Memory device 702 can be constructed as an integrated circuit, whichincludes one or more memory arrays 100 and/or diodes 10 a, 10 b. Ifdesired, the memory device 702 may be combined with the processor, forexample CPU 710, as a single integrated circuit.

The above description and drawings should only be consideredillustrative of exemplary embodiments that achieve the features andadvantages described herein. Modification and substitutions to specificprocess conditions and structures can be made. Accordingly, the claimedinvention is not to be considered as being limited by the foregoingdescription and drawings, but is only limited by the scope of theappended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of forming a diode comprising: forminga layer of a first conductive material; forming a heating element overthe first conductive material; forming a layer of a diode material overthe heating element; and forming a layer of a second conductive materialover the diode material, wherein the diode material is selected from thegroup consisting of a vanadium oxide, an aluminum-doped vanadium oxide,a titanium-doped vanadium oxide and a rare earth manganate representedby the formula Ln_((1-x))A_(x)MnO₃, wherein Ln represents a rare earthelement and A represents an alkaline earth element.
 2. The method ofclaim 1, wherein the diode material is La_((1-x))SR_(x)MnO₃, where0.6≦×≦1.0.
 3. The method device of claim 1, wherein the diode materialis VO₂.
 4. A method of forming a diode comprising: forming a firstconductive material; forming a heating element over the first conductivematerial; forming a diode material over the heating element; and forminga second conductive material over the diode material, wherein the diodematerial has characteristics allowing a first decrease in a resistivityof the diode material upon application of a first voltage to the diodematerial, thereby allowing current to flow through the diode material,and having further characteristics allowing a second decrease in theresistivity of the diode material in response to a temperature increaseof the diode material by joule heating.
 5. The method of claim 4,wherein the temperature increase of the diode material is due to heatgenerated by current flowing through the diode material.
 6. The methodof claim 4, wherein the temperature increase of the diode material isdue to heat generated by the heating element.
 7. The method of claim 4,wherein the diode material comprises a vanadium oxide.
 8. The method ofclaim 4, wherein the diode material comprises VO₂.
 9. The method ofclaim 4, wherein the diode material comprises an aluminum-doped vanadiumoxide.
 10. The method of claim 4, wherein the diode material comprises arare earth manganate represented by the formula Ln_((1-x))A_(x)MnO₃,wherein Ln represents a rare earth element and A represents an alkalineearth element.
 11. The method of claim 4, wherein the diode materialcomprises La_((1-x))Sr_(x)MnO₃, where 0.6≦×≦1.0.
 12. The method of claim4, wherein the first decrease in the resistivity of the diode materialoccurs at a voltage of approximately 0.5V to approximately 1.5V.
 13. Themethod of claim 4, wherein the second decrease in the resistivity of thediode material occurs at voltage of approximately 1.5V to approximately4.0V.
 14. A method of forming a memory device comprising: forming amemory element; and forming a diode for accessing the memory element,wherein forming the diode comprises: forming a first conductivematerial; forming a diode material over the first conductive material;forming a heating element over the diode material; and forming a secondconductive material over the heating element, wherein the diode materialis selected from the group consisting of a vanadium oxide, analuminum-doped vanadium oxide, a titanium-doped vanadium oxide and arare earth manganate represented by the formula Ln_((1-x))A_(x)MnO₃,wherein Ln represents a rare earth element and A represents an alkalineearth element.
 15. The method of claim 14, further comprising forming aplurality of memory elements and a plurality of corresponding diodes foraccessing respective memory elements, wherein each diode is a selectdevice for a corresponding memory element, and wherein the memory deviceis a cross-point memory.
 16. The method of claim 14, wherein the diodematerial has characteristics allowing a first decrease in a resistivityof the diode material upon application of a first voltage to the diodematerial, thereby allowing current to flow through the diode material,and having further characteristics allowing a second decrease in theresistivity of the diode material in response to a temperature increaseof the diode material by joule heating.
 17. The method of claim 16, thetemperature increase causing the second decrease in the resistivity ofthe diode material is due to at least one of heat generated by currentflowing through the diode material and heat generated by the heatingelement.
 18. The method of claim 14, wherein the diode material is VO₂.19. The method of claim 14, wherein the diode material is analuminum-Doped vanadium oxide.
 20. The method of claim 14, wherein thediode material is La_((1-x))Sr_(x)MnO₃, where 0.6≦×≦1.0.